Laser ablation has long been used in the ophthalmic and semiconductor fields. In the past, for example, photolithographic processes were used to produce the vias that connect different levels of circuitry of semiconductor chips. Laser ablation processes have replaced the photolithographic processes in some instances and are now used for ablating the via patterns. In the optical field, on the other hand, laser ablation has been used for shaping contact lenses, such as smoothing their surfaces.
Many obstacles are present when using laser ablation processes, such as spot writing with a shaped beam, shadow mask systems, projection mask systems and phase mask systems. For those systems that use a mask, it is expensive and inefficient to provide perfectly uniform illumination of the mask. The distributions of the beam intensity from an excimer laser is roughly rectangular in cross-section. An ideal beam has a top-hat profile in the long dimension and a Gaussian cross-section in the short dimension. The non-uniform beam intensity can create nonconformities in an ablated substrate. FIG. 1a is a graphical representation of the energy profile of a typical excimer laser beam in cross-section. While the intensity of the beam is substantially uniform in the center portions of the beam cross-section, the intensity of the beam drops off near the beam edges. The nonuniformity of the beam intensity can translate to nonuniformities in the ablated materials, such as less material removal and formation of spurious posts due to contamination in the material being ablated in lower intensity areas and straighter walls in the higher intensity areas. Therefore, methods and apparatuses have been developed to overcome the nonuniformities inherent in the beam intensity.
Beam homogenizer systems have been developed to improve the uniformity of the cross-section of laser beams. FIG. 1b is a graphical representation of the energy profile of the excimer laser beam shown in FIG. 1a in cross-section after passing through a beam homogenizer. Beam homogenizers output a substantially constant energy beam across a mask plane, thereby providing more even ablation across a substrate. Beam homogenizer systems included in laser ablation systems increase the cost of the laser ablation system, however, and therefore can be undesirable in lower cost systems. Moreover, beam homogenizer systems are less efficient than non-homogenized beams due to energy losses in the system and require more pulses to ablate the same amount of material because of the lower pulse energy.
Laser ablation systems, such as projection systems utilizing lenses, have additional problems creating uniform morphologies. Projection laser ablation systems use a projection lens to transfer an image of the desired pattern from the mask plane to the substrate plane. This image transfer is never perfect due to spherical and chromatic aberrations, coma, astigmatism, field curvature and distortion, and other higher order effects on the wavefront. These distortions can cause nonuniformities in the ablated substrates, thereby reducing the precision available to create uniform, micromachined surfaces. To partially overcome lens aberrations, higher quality, higher cost lenses may be used in projection systems. These high cost lenses, however, cannot be justified in all projection systems.
The laser ablation process can generate a substantial amount of debris from the ablated substrate, thereby causing additional obstacles to precision machining or uniform morphologies in the ablated substrate. When features are closely spaced and a large area is to be covered, some debris typically falls on areas that are to be later ablated. Substrates that are covered with a large amount of debris do not have the same ablation characteristics as clean substrates or substrates that are covered with only a small amount of debris. Particularly when ablating larger features, the generated debris can cause interference with the current or subsequent ablation sites. Debris removal systems have been developed for use with laser ablation systems to remove the debris remaining on the substrates to be ablated to minimize the amount of debris the laser beam must penetrate to sufficiently ablate the surface of the substrate. One method of debris removal utilizes assist gases, such as helium or oxygen. Performing the ablation in a vacuum also reduces debris, but further adds complexity to the system. With assist gases or vacuums incomplete debris removal can occur, particularly with larger features, thereby resulting in residual formations.
Because a very high fluence is required to ablate material from a substrate, the area of the image field at the substrate is typically quite small, on the order of less than one square centimeter. Typical step and repeat processes allow production of a large number of images, the images being far removed from each other, such that debris produced by the process and nonuniformities between the images are not an issue. In order to cover large areas on a substrate, however, step and repeat processes or a mask scanning operations may be performed. In a step and repeat process, a first image is exposed to the necessary number of laser beam pulses such that the substrate is ablated to the desired depth. Then, the substrate is moved such that an unexposed area of the substrate is in the image field, preferably so a second image may be exposed immediately adjacent the first image. This process is repeated until the desired area is covered with the repeated images.
When step and repeat processes are used to cover large areas of a substrate, however, nonuniformities become an issue because it is difficult to hide the intersections between adjacent images over a large area of the substrate. FIG. 2 shows an enlarged perspective view of a substrate having a pattern created with step and repeat imaging. The repeating pattern of square holes on the substrate may be produced by ablating adjacent square images having the repeating pattern of square holes. Intersection point 10 is the intersection of four images. In one image, walls 11 underexposed, thereby leaving polyimide residue on the top of the walls. In a second image, walls 12 were overexposed, thereby removing too much polyimide and leaving incomplete walls. Very slight differences in a pattern caused by, for example, the aforementioned nonhomogeneous beam profile, minor distortion in the image or attempting to ablate through the debris left by ablating the adjacent image is visible to the human eye. In mask scanning operations, a large area mask, with a pattern on it corresponding to the entire area to be covered is used and the mask is moved synchronously with the substrate. These large area masks, however, are very expensive to create.
Laser ablation systems can also be used for micromachining. Microelectronics and micromechanics require production technologies to produce small structures and small parts. Laser ablation is well suited for precision production of small, precision structures, particularly applications requiring drilling, cutting, material removal and surface modification of materials. Excimer lasers have been used to machine metals, ceramics and polymers when small structures are required. For example, in a spot writing system, an excimer laser may be used as a stylus, where the laser beam has an ablating resolution of one micron. While this type of single spot writing system allows ablation of three-dimensional structures, the laser typically operates around 2000 Hz. The rate at which the surface area is ablated is slow, thereby making this method impractical for covering large areas.
To ablate larger three-dimensional geometries, the three-dimensional geometry may be separated into slices parallel to the x-y plane. The thickness of each slice is equivalent to the removal depth of one or more laser pulses. Beginning with the largest mask, the surface associated with each slice is removed with a single mask. The process is continued with smaller and smaller mask size until the three-dimensional geometry has been created. The aforementioned method of creating three-dimensional structures are either expensive, have a limited application or are inefficient. For example, precision control mechanisms exist for moving the laser beam or the mask to precisely position the beam or mask relative to the substrate. Such accurate positioning allows precise ablation of a slice of the three-dimensional structure. Other costly and inefficient methods place the entire pattern on a single mask and shutter off unused portions of the mask, thereby not using all the light from the laser or require multiple masks to create the entire three-dimensional pattern.